Respiratory systems provide breathable gas, such as oxygen, anesthetic gas and/or air directly to a patient's mouth, nose or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit of a breathing circuit. The breathing circuit may include an expiratory limb hose or conduit to carry expelled air and other gas(es) from the patient back to the ventilator.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a humidification system having a heater unit and a disposable water chamber adapted to be heated by the heater unit. The heater unit supports a hot plate heater, which may be comprised of one or more heating elements and a metal plate defining a hot plate. A wall of the chamber, such as the bottom surface of the chamber, is thermally conductive. The chamber is removably supported on the heater unit with the bottom surface in thermal contact with the hot plate of the heater unit to thus heat the water in the chamber. The chamber may be manually refillable, or there may be a water source to selectively fill the chamber as it empties. The breathable gas is coupled to the chamber and is passed through the chamber to be heated and humidified. The inspiratory limb carries the heated and humidified gas to the patient and the expiratory limb, if present, carries exhaled air and possibly other gases from the patient. Examples of heater units, chambers and vented water supplies are shown in U.S. Pat. Nos. 6,988,497 and 5,943,473; and co-pending U.S. patent application Ser. Nos. 11/469,086 and 11/469,113, both filed Aug. 31, 2006.
The hoses or conduits of the inspiratory and expiratory limbs may each be provided with a heater circuit to add heat to the gas passing through the limb. The heater circuit may be in the form of one or more elongated, and possibly coiled, heater wires running along the limb, such as through the interior of the limb. An example of a breathing circuit with heated limbs is shown in U.S. Pat. No. 6,078,730. The heater unit typically houses the necessary electrical and electronic components to regulate the temperature of the hot plate, as well as heating circuits of the inspiratory and/or expiratory limbs of the breathing circuit. To that end, the temperature of the gas passing through the breathing circuit may be monitored at various locations, two examples of which are at the outlet of the chamber (i.e., the inlet to the inspiratory limb) and/or at the outlet of the inspiratory limb (i.e., at the patient). The temperature of the hot plate may also be monitored. Those temperatures may each be monitored with a respective temperature responsive device that provides temperature readings as feedback to the heater unit for purposes of regulating the various heating components. The temperature responsive device may take the form of a thermistor coupled to the hot plate or held within a probe adapted to be inserted into the flow path of the gas through the limb.
Resistance of a thermistor varies in relation to the temperature to which the thermistor is exposed. Detection circuitry coupled to the thermistor can sense the resistance, such as by sensing a voltage induced across the thermistor by a constant current passing therethrough, and provide an output signal, such as in the form of a digital temperature signal or word, corresponding thereto (and thus to the temperature at the thermistor). The digital temperature signal may be utilized by a processor to generate a useful form of the digital temperature signal which should be correlated to the actual sensed temperature, such as by undertaking appropriate calculations or utilizing a look-up table(s), which produce the useful form of signal utilized by the processor or other circuitry of the heater unit to monitor and/or control the various heaters of the humidification system. The detection circuitry and processor thus cooperate to define temperature determining circuitry adapted to convert temperature readings from the thermistor to a useful form for regulating a source of heat, namely one or more of the heaters of the humidification system.
Typical components of the detection circuitry include an operational amplifier to amplify analog temperature readings from the thermistor, and an A/D converter to convert the amplified signals to the digital temperature signals or words. The temperature signal may be coupled to the + input of the amplifier. The operational parameters of the amplifier are typically set by resistors so as to have an offset and a gain determined by the impedance of the resistors. For example, a feedback resistor(s) interposed between the − input and the output of the amplifier and one or more of a tie-up (to the positive power supply rail) and/or tie-down (to the ground or negative power supply rail) resistor coupled to the − input of the amplifier sets the gain and offset of the amplifier. Ideally, the resistors are selected to set the amplifier to produce an output that can extend over the full extent of voltage in a generally linear range (or at least over a range that avoids an undesirably non-linear operating region) for the expected range of voltage at the + input.
By way of example, the resistance of a thermistor expected to encounter a range of temperatures between a low temperature and a high temperature will vary from an expected high impedance to an expected low impedance, respectively. The offset and gain of the amplifier are typically set such that the amplifier will output signals from about 0 volts to about 2.5 volts (by way of example) across the expected temperature range. The gain and offset may be provided by fixed resistances, or one or both may be manually adjusted such as where a selected resistance is provided by the setting of a potentiometer or the like. However, the components of the detection circuitry, such as the amplifier and associated resistors if not also the A/D converter, can drift over time such that the digital temperature signal may become inaccurate, or various of those components could fail altogether. Erroneous or otherwise useless data may thus result, leading to inaccurate or improper operation of the heater unit. As a result, it is customary to periodically take the heater units out of service for re-calibration by a technician, for example.
Conventional re-calibration may involve taking the heater unit to a workshop or other location, where it can be hooked up to test equipment to test operation of the detection circuitry. If the components have drifted, the technician may be able to at least partially restore the desired operating conditions such as by replacing some of the components or adjusting those that can be varied in situ, such as one or more potentiometers or the like. The re-calibrated heater unit is then returned to service. Conventional re-calibration of the heater unit presents a number of disadvantages. Often, the range over which adjustments can be made, such as of the potentiometers, is insufficient to restore the original operating range. Further, re-calibration may not always result in obtaining proper settings. Moreover, the heater unit may be operating for some period of time while not properly calibrated. Additionally, heater units may be taken out of service on a regular re-calibration schedule, even if it turns out that, on testing, the unit is still within specification. Thus, conventional re-calibration adds to cost and resource demands to users, not to mention the risk of delay in providing desired operation of a respiratory system.